Autolite 4100 Evaporative Emissions Experiment

[LARIAT 85]

I don't think the charcoal canisters "pull" or have any sort of vacuum.  

I think what happens is, as the gas in the gas tank (or carburetor) expands, the excess gas fumes are routed to the charcoal canister. The charcoal absorbs the fuel vapors and holds them in the canister, while the air is allowed to exit the canister through the mushroom cap on the top of the canister.

When the engine is running - and when the right conditions are met - a ported vacuum switch or engine computer controls when the purge valve opens.  When the purge valve opens, the PCV valve is allowed to pull (purge) the fuel vapors that are trapped within the charcoal canister(s) and into the engine to be burned.

[LARIAT 85]

Alright, let's forget the cold start issue for a moment and look at this from another angle.

With the engine running and completely warmed up with the evaporative emissions in place, I had to richen the mixture screws by about a 1/2 turn to get my engine back to my normal 20.5" of vacuum.  And the curb idle speed was down from the usual 750RPM in PARK to 500 RPM in PARK, so I also had to turn the curb idle speed screw in about a 1/2 to get it back to my normal idle speed.  I don't have a dyno or anything - so I am not 100% sure on this - but the engine seems down on power as well.

But why?  What is happening here?  When the engine is running with the modified lid and evaporative emissions in place, the difference is the external bowl vents are CLOSED, thanks to the bowl vent valve blocking the hose to the canister.  The only place venting is done is inside the air cleaner by the four (internal) bowl vents.


Why does it run different (better) when the Autolite 4100 is completely stock and the external bowl vents are open to atmosphere when the engine is running?

[Gary Lewis]

I agree - divide and conquer.  One issue at a time.

So, when the engine is running the external bowl vents are closed.  But I don't see in the proceeding dialog what the internal bowl vents are like.  Are there still vents under the air cleaner?  If not, then you don't have any venting when it is running.

Also, why did Autolite put those small external vents in - the ones you closed?  My guess is that they are needed and w/o them there's a vacuum or pressure on top of the fuel when running.  (Perhaps pressure as the air is rushing in through the air cleaner and probably into the bowl as well - assuming there are internal vents.

So, I'm back to my suggestion: pull the hose off the fitting you installed and see if it runs right with the area in front vented.

[LARIAT 85]

There are FOUR (4) internal vents that always vent to the air cleaner.  The emissions Motorcraft 2150 and Motorcraft 4180 carburetors also have these.  In fact, all carburetors have these.  I left them open.  They are open at all times, whether the engine is running or not.

All I did with the modified lid was to stop the venting *outside* of the air cleaner.  Instead of excess fuel vapor venting out to into the atmosphere at all times, it now vents to the charcoal canister when the engine is off to contain the fumes.  When the engine is running, it is only allowed to vent *inside* the air cleaner - just like the stock emissions carburetors.  

Why were those external vents there to begin with?  When the engine is hot, what happens to the fuel supply that is stored in the carburetor fuel bowl?  It expands.  When it expands, that pressure has to go somewhere.  Before emissions were a concern, they were allowed to simply vent out into the atmosphere through the external bowl vent(s).  If there were no external bowl vent(s), all of that pressure and those excessive gas fumes could only vent inside the air cleaner.  The problem with that is, it creates an overly-rich fuel mixture inside the air cleaner, in which case hard hot starts can occur.  


To follow your thinking, if there is pressure as the air is rushing in through the air cleaner (and probably into the bowl as well), that pressure would find it's way out through the external vents on the stock Autolite 2100/4100 arrangement.  But what happens when that vent is blocked?  Does something different happen to the fuel in the fuel bowl?

[Gary Lewis]

LARIAT 85 wrote

To follow your thinking, if there is pressure as the air is rushing in through the air cleaner (and probably into the bowl as well), that pressure would find it's way out through the external vents on the stock Autolite 2100/4100 arrangement.  But what happens when that vent is blocked?  Does something different happen to the fuel in the fuel bowl?

Yes.  If there is rushing air hitting those vents and no external vent where it can escape then the pressure goes up inside the bowl.  And that will change the air/fuel ratio the carb meters.

You seem reluctant to do my test.  But it sure looks easy to me, and would prove or disprove my theory.  But if pulling the vent line itself is hard then pull and plug the vacuum line that closes the valve.

[LARIAT 85]

I would be more than happy to try your easy test.  But like I said in my first post, I already switched the lids back and put everything back to normal.  I won't be able to switch them back and try it again until the weekend.  And anyway, that has nothing to do with the engine running with the bowl vent closed.


Early emissions had only a thermal vent valve in the carburetor-to-canister hose, which means the vent hose was blocked anytime temperatures were below 90 degrees, whether the engine was running or not.  Those carburetors worked fine like that.

Later emissions had both a thermal vent valve AND the bowl vent valve in the carburetor-to-canister hose, which ensured the vent was blocked any time the engine was running, regardless of temperature.  Those carburetors worked fine like that.

In every case, these emissions carburetors also had internal air cleaner vents...just like my non-emissions Autolite 4100.


Both non-emissions carburetors AND emissions carburetors had external bowl vents.  The only difference was, non-emissions carburetors vented to the atmosphere, while emisssions carburetors vented to a charcoal canister.

The emissions carburetors had no moving parts other than the choke rod and no other open ports or valves on the carburetor lid...just like the lid on my non-emissions Autolite 4100.



If pressure goes up inside the fuel bowl, you said it will "change the air/fuel ratio the carb meters."  I am intrigued.  Can you elaborate?  I think that will tell me more and be a much more interesting conversation than me swapping the modified lid back on just to pull the vent hose back off.

[Gary Lewis]

LARIAT 85 wrote

If pressure goes up inside the fuel bowl, you said it will "change the air/fuel ratio the carb meters."  I am intrigued.  Can you elaborate?  I think that will tell me more and be a much more interesting conversation than me swapping the modified lid back on just to pull the vent hose back off.

Princeton says "The lower pressure in the venturi will attract fuel from the fuel bowl into the pipe in an attempt to even out of the lower vapor pressure of the venturi and the higher atmospheric pressure in the fuel bowl."

So it is the differential between the pressure on the fuel in the bowl and the pressure seen in the venturi that causes the flow.  If you change the pressure on the fuel in the bowl you will change the flow of fuel for a given flow of air in the venturi.  In other words, you will alter the air/fuel ratio.

I can't tell you that is what is happening, but I know for sure that if you change the pressure you will change the AFR.  And since air has mass, if air flowing into the air cleaner has a straight shot into the bowl via a vent then the pressure will go up.

[FuzzFace2]

Gary Lewis wrote

LARIAT 85 wrote

If pressure goes up inside the fuel bowl, you said it will "change the air/fuel ratio the carb meters."  I am intrigued.  Can you elaborate?  I think that will tell me more and be a much more interesting conversation than me swapping the modified lid back on just to pull the vent hose back off.

Princeton says "The lower pressure in the venturi will attract fuel from the fuel bowl into the pipe in an attempt to even out of the lower vapor pressure of the venturi and the higher atmospheric pressure in the fuel bowl."

So it is the differential between the pressure on the fuel in the bowl and the pressure seen in the venturi that causes the flow.  If you change the pressure on the fuel in the bowl you will change the flow of fuel for a given flow of air in the venturi.  In other words, you will alter the air/fuel ratio.

I can't tell you that is what is happening, but I know for sure that if you change the pressure you will change the AFR.  And since air has mass, if air flowing into the air cleaner has a straight shot into the bowl via a vent then the pressure will go up.

That is what I was thinking, the air rushing in to the carb is also going down the bowl vent tubes raising the pressure in the bowl causing the AFR change.
You would think it would make it run rich so I cant say why it is lean?
Could it be the air bleed jets why the change?
Dave ----

[Gary Lewis]

FuzzFace2 wrote

That is what I was thinking, the air rushing in to the carb is also going down the bowl vent tubes raising the pressure in the bowl causing the AFR change.
You would think it would make it run rich so I cant say why it is lean?
Could it be the air bleed jets why the change?
Dave ----

Yes, I would think that it would make it run rich because the pressure in the bowl probably goes up, so the differential should be higher and, therefore, the mix would be richer.  However, the symptoms sound like it is lean.  So, perhaps my analysis is wrong?

This article from Holley has this bit of info: "Most carburetors will have a vent tube that acts as a “port” to the fuel bowl; this “port” provides the carburetor with pressure from it’s ambient surroundings and forces the fuel to move through the metering passages as required based on engine demand. Manipulation or changing the length of the fuel bowl port can have dramatic affects on the fuel curve of a carburetor and should only be done with the assistance of a professional engine dyno."

[LARIAT 85]

Interesting links and observations.  Thank you, gentlemen.

"Manipulation or changing the length of the fuel bowl port can have dramatic affects on the fuel curve of a carburetor" is right.  My truck went from starting and running stupid easy and flawlessly to a complete bear to start and struggling to run.

What else is different?  Like 85lebaront2 said, the later Motorcraft 2150 fuel bowl vent ports in the air cleaner are much bigger than the tubes found on the earlier Autolite 2100/4100 airhorn:



Perhaps the larger, shorter fuel bowl vent ports somehow work in conjunction with the larger external bowl vents and evaporative emissions?  And/or the internal calibration is different as a result?  If you look at an early Autolite 2100 or 4100, the fuel bowl vent ports are much taller, and slimmer.  And the external vent hole is also much smaller:




The internal and external venting of a carburetor is apparently more important and affects much more than I thought.  It seems there is much more going on with the inner workings of the stock carburetors than you might would think.  I thought it would be a simple upgrade, but apparently I was wrong.  Trying to upgrade the earlier non-emissions carburetor this way only throws off the balance of the carburetor.  

[Gary Lewis]

I agree, whole heartedly.  There are complicated things going on that I don't understand, for sure.  Like, are the diameters of the vents, or their heights significant?  Do the vents just end straight down, or do they turn?  Is there an orifice at the end of the vent and how big is it?  Why is the external vent tiny?

The carbs appear to be tuned for the answers to those questions, and many more besides.  And you just happened to change something that makes a noticeable difference.

[81-F150-Explorer]

Rick has asked me to look into this thread for any insights.

Four things come to mind with what you are trying to do.

1: The Carburetor needs to vent from atmosphere into the bowl when the engine is running. Is there a stipulation for that? It also needs this vent to be one way, into the bowl. It can't be both ways etc. The evaporative carbs have a internal check valve to keep fuel vapor from escaping the bowl in any other way than the bowl vent, unless extreme pressure buildup happens.

2: The Bowl Vent hose also needs a one-way thermal check valve.  D8TZ-9E589-B  Motorcraft CX-233.
This also needs to be hooked up inline with the word "CARB" toward the carb bowl vent.

3: There needs to be a purge control valve to control when the canister purge operation happens.  D9AZ 9B963-B Motorcraft CX-330.

4: The old carb configuration lets fuel vapor vent into the air cleaner. When the vehicle is started in the mornings this air in the cleaner is rich. This might help with starting in your original configuration.


So for evaporative emissions to work correctly, you will need:

1: An air vent into the carb bowl from the outside, that only allows air to enter the bowl, and not to escape. The biggest problem is your carb isn't set up for evaporative emissions so does not have these provisions.

2: From the bowl vent hose/s the one-way thermal check valve D8TZ-9E589-B with the "CARB" toward the bowl vent. This valve needs to open at the right temp, and only allow vapor through one-way.

3: From the Check Valve to the Canister Purge "T" fitting between the Canister and the PCV valve.

4: Between the "T" fitting and the PCV valve the canister control valve D9AZ 9B963-B Motorcraft CX-330. hooked up to an appropriate vacuum source for proper purge operation.

5: The Charcoal Canister itself must be able to vent clean air into itself, when in purge mode.

6: The fuel tanks fuel caps must be able to vent clean air, one way into the tank.

Any one of these can cause a fault or drivability issue if not working properly.